A serious problem in the development of submillimeter wavelength receivers for atmospheric and space spectroscopy is the availability of suitable local oscillators required for heterodyne mixing. Such local oscillators must have reasonable power output and efficiency, and are required to cover a range of wavelengths of interest to spectroscopy. Lasers are being developed for this purpose, but each laser system is restricted essentially to a single wavelength. Some tunability can be achieved by optical techniques, but only over very limited bandwidths. Microwave generators such as carcinotrons do not operate efficiently at wavelengths shorter than a millimeter, and they are excessively heavy, power consuming and of short lifetime, restricting their use in flight missions. Available solid-state oscillators, such as GaAs Gunn diodes and IMPATT (impact ionization avalanche transit time) diodes are highly efficient and tunable, but are currently limited to frequencies up to about 75 and 150 GHz, respectively, for output power .gtoreq.0.1W.
Much higher frequencies can be obtained by generating harmonics of the fundamental frequency from these available solid-state oscillators. Two cascaded harmonic multipliers based on whisker-coupled GaAs varactor diodes in waveguide configurations have produced 0.3 mW at 492 GHz. However, waveguide fabrication and impedance matching technologies are already at their limits at this frequency. Therefore, planar structures for quasi-optical coupling are preferred. In the planar technology, arrays of diodes can be easily fabricated and integrated with antenna structures. The total power produced is then proportional to the number of diodes provided. However, varactor diodes have serious limitations at higher frequencies. These stem primarily from the parasitic resistance introduced by the front ohmic contact. Furthermore, the weak dependence of the capacitance on the voltage C(V) limits efficiency in harmonic generation, especially for higher harmonics.
Another example of this planar array approach, which overcomes the deficiencies of the varactor diodes involves T-MOS (thin metal-oxide-silicon) diodes, which have an undoped thin epitaxial silicon layer, and exhibit an exponential dependence of the space charge capacitance on voltage, and thus produces harmonics more efficiently. This has been demonstrated with single diodes in a whisker-coupled waveguide configuration for frequency doubling and tripling. Furthermore, due to the blocking barrier, two diodes can be operated back-to-back generating a sharp spike in the C(V) curve. This arrangement, which needs no external ohmic contact, makes a highly efficient frequency tripler in which the efficiency does not degrade with high fundamental power. As a further advantage, the input and output impedances are doubled. However, defects in the epitaxial silicon layer deteriorate the thin oxide and limit the fabrication yield of the device.
Yet another planar array approach investigated involves monolithic Schottky diode grids fabricated on 2-cm square gallium-arsenide wafers. Second harmonic conversion efficiencies of 9.5% and output powers of 0.5 W were achieved at 66 GHz when the diode grid was pumped with a pulsed source at 33 GHz. However, using currently realizable diode parameters, it should be possible to achieve CW doubling efficiencies of 60% at 66 GHz with output powers of 2 W using edge cooled wafers.